The present invention generally relates to a manufacturing method for an optical type recording and reproducing disc and more particularly, to a method of forming hard coating films on outer surfaces of discs.
In conventional optical type recording discs, it has been a general practice that after a process of forming a recording layer enabling optical recording and reproducing, on a plastic plate of a circular shape, has been completed, another plastic plates are attached to the recording layer as required so as to protect the recording layer against damage. However, the known optical type recording discs present an inconvenience in that the plastic plates for protecting the recording layer from damage also sustain flaws on the surfaces through contact with dust or improper materials during manufacture. The above described disadvantage also presents a serious problem to optical type recording and reproducing discs utilizing laser beams; namely the flaws produce noises in recording and reproducing signals.
In this connection, the present inventors have disclosed in U.S. patent application No. 174,600, filed Aug. 1, 1980 and U.S. patent application No. 247,085, filed Mar. 24, 1981 that coating compositions are coated and cured on the optical type recording and reproducing disc manufactured by a conventional method so as to form a transparent hard coating film which exhibits an outstanding effect for preventing flaws. However, the coating compositions disclosed in the above described U.S. Patent Applications have posed a new problem in that it is difficult to form a uniform coating film and application of the coating compositions to a one-sided coating disc obtained by the conventional horizontal spinner method causes the disc to be distorted. Furthermore, the above described coating compositions have a disadvantage in that when the disc is stopped temporarily after a spinner operation so that ultraviolet rays may be irradiated onto the coating film for curing thereof, a small difference in coating film thickness arises between peripheral portions and a central portion of the disc. More specifically, the coating compositions have been disadvantageous in that the disc having a large distortion or the cured coating film of greatly non-uniform thickness makes it difficult to maintain a constant clearance between a semiconductor laser element and the disc, so that it becomes impossible to perform recording and reproduction of signals, thus resulting in deterioration of performance of the disc.